feat: add miniPaint stack (paint.pivoine.art)

Added new paint service stack to the docker-compose infrastructure:

- **Paint stack** (paint.pivoine.art):
  - miniPaint: Web-based image editor built from GitHub
  - Multi-stage Docker build clones from https://github.com/viliusle/miniPaint
  - Features: layers, filters, drawing tools, text, shapes support
  - Client-side processing with no server uploads
  - Stateless architecture (no backups needed)

Infrastructure updates:
- Created paint/compose.yaml with Traefik routing and SSL
- Created paint/Dockerfile with Node.js build stage and nginx serve
- Added PAINT environment variables to arty.yml
- Updated compose.yaml include list
- Updated CLAUDE.md documentation

All services integrated with Traefik for SSL termination and include
Watchtower auto-update labels.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

paint/Dockerfile

@@ -0,0 +1,23 @@
+# Build miniPaint from GitHub repository
+FROM node:18-alpine AS builder
+
+WORKDIR /app
+
+# Clone the repository
+RUN apk add --no-cache git && \
+    git clone https://github.com/viliusle/miniPaint.git . && \
+    npm install && \
+    npm run build
+
+# Production stage with nginx
+FROM nginx:alpine
+
+# Copy built files from builder
+COPY --from=builder /app/dist /usr/share/nginx/html
+
+# Copy nginx configuration if needed
+COPY --from=builder /app /usr/share/nginx/html
+
+EXPOSE 80
+
+CMD ["nginx", "-g", "daemon off;"]